The present invention is related to a fiber optic sensing system and particularly to one enabling an environmental parameter to be sensed using a miniaturized sensing tip at one end of an optical fiber.
Optical fiber sensing systems can be used for a variety of applications. For example, the measurement of intravascular blood pressure of human patients has been accomplished using equipment manufactured by applicants in which a diaphragm at the fiber sensing tip deforms in response to a pressure differential and modulates light sent through the fiber in accordance with its deflection. Changes in the distance between the deformable diaphragm and the optical fiber tip changes the amplitude of light that is reflected back into the optical fiber. Accordingly, the intensity of the returned light is related to the sensed pressure. Numerous other applications for such systems exist, such as for measuring pressure of other fluids, measurement of temperature, index of refraction, etc.
Although applicants have had excellent success in implementing fiber optic sensors of the above type for intravascular blood pressure measurement, additional refinements for such system are being strived for. In medical applications, it is preferred to employ a disposable optical fiber with a sensing tip which is used as a catheter. To minimize the expense of using such a system, it should accommodate the use of relatively inexpensive plastic or glass optical fiber materials while providing acceptable accuracy and sensitivity. In order to enable the use of such low cost fibers, the system must accept a large degree of fiber-to-fiber variability in index of refraction and attentuation, and fiber bending sensitivity.
In applicants' previously designed system as described in issued U.S. Pat. No. 4,711,246 which is hereby incorporated by reference, the catheter is placed in position and a precisely controlled pressure signal is applied through the fiber to one side of the diaphragm, which produces deflection of the diaphragm. The intensity of the returned light is evaluated and used for calibration of the system. Thereafter, the measured intensity signal is compared to a software look-up table to generate an output indicating pressure at the sensing tip of the catheter.
Although the previously described calibration approach is capable of eliminating or reducing the effects of many sources of measurement inaccuracy, many contributions to inaccuracy are not addressed. Optical fibers in general, and particularly low cost plastic and glass fibers, exhibit a substantial change in light attenuation dependent upon their bending radius. In intravascular blood pressure measurement, even after the calibration steps, dynamic bending can occur due to pressure pulses or movement of the fiber inside or outside of the patient. In addition, certain optical fiber couplers exhibit changes in their characteristics due to mechanical inputs during use. Other sources of measurement inaccuracy or noise are the time dependent characteristics of the light sources and photodetectors (i.e., changes in intensity or sensitivity) which are used with such a measurement system. Due to these time dependent factors, it is present operating procedure with current amplitude based fiber optic systems to allow the electronics to reach a steady state operating temperature before use, which may require one hour or more. Such warm-up time requirements can be intolerable in some conditions such as for emergency surgery.
In view of the foregoing, it is an object of the present invention to provide a fiber optic sensing system which is essentially self-compensating in nature to reduce the effects of measurement noise generated by the previously mentioned factors.
In addition to the need to reduce measurement noise, designers of optical fiber sensing systems using deflectable diaphragms are further striving to enhance the sensitivity of such devices by increasing the changes in returned light intensity in response to deflection of the diaphragm. Accordingly, it is another object of the present invention to provide an optical fiber sensing system which provides enhanced measurement sensitivity.
For fiber optic sensors of the type described previously in which an input light signal is transmitted through the fiber and returned after being modulated in some manner, it is necessary to inject the input light signal into one end of the optical fiber and evaluate the returned signal at the same end of the fiber. Typically, beam splitters in the form of planar partially reflective mirrors are used for this purpose. Such beam splitter systems, however, are inefficient in terms of coupling the full output of the light source into the fiber and further do not provide a high degree of isolation of the returned signal from the input signal at the photodetector.
It is therefore another principal object of the present invention to provide a beam separator for a fiber optic sensing system which enhances the isolation of the returned light signal from the input light signal.